1. Field of the Invention
This invention relates to signal receiver and more particularly a signal receiver with a gain controller that controls a gain of its amplifier for received signal, the signal having a time interval containing a valid data and a time interval containing an invalid data.
2. Description of the Related Art
In Europe, a broadcast system called a DAB (Digital Audio Broadcast) system is now in operation. In this DAB system, an OFDM (Orthogonal Frequency Division Multiplex) modulation system that is one of multi-carrier digital modulation technique is employed for generating a broadcast signal.
In this OFDM modulation system, a modulated signal comprises a plurality of carriers having orthogonal relations each other. For example, in case of the DAB system, an audio signal is encoded at first, and thus encoded audio signal is allocated to each of a plurality of carriers for modulation. This digital signal comprised of a plurality of modulated carriers in a frequency domain is converted into a digital signal in a time domain by applying the Reverse Fourier Transform and this digital signal of the time domain is converted into an analog signal. Thus converted analog signal is transmitted as the broadcast signal.
In a signal receiver of the DAB system, a received modulated signal is converted into a digital signal and then the Discrete Fourier Transform is applied to this converted digital signal. Resultantly, an audio data allocated to each carrier is provided by this Discrete Fourier Transform.
A signal transmission by the OFDM modulation system is done by a sequence of an unit called a symbol as shown in FIG. 9A. In the signal receiver of the DAB system, when a received data signal in this FIG. 9A is demodulated, the received data signal is amplified with an AGC (Automatic Gain Control) processing at an intermediate frequency signal stage.
In the case of a DAB receiver which receives the broadcast signal of the DAB system, the AGC operation is controlled by a digital signal processing. A gain of an amplifier at the intermediate frequency signal stage is changed in step by the digitally controlled AGC as shown in FIG. 9B relative to the received data signal. In the case when the gain of the amplifier at the intermediate frequency signal stage is changed in incremental steps by the digitally controlled AGC as shown in FIG. 9B, an output intermediate frequency signal obtained from the intermediate frequency signal stage includes distortions at points (depicted in FIG. 9A by arrows with dot-lines). These distortions occur due to abrupt level change in the output intermediate frequency signal. Namely in this case, a fidelity of data obtained from the demodulated signal becomes low and error rate of the recovered data becomes high.
In order to avoid above defects, an AGC system of high quality is required. Such AGC system of high quality requires more high grade digital signal processing for the AGC system, and accordingly the cost for the signal receiver becomes high.
In order to overcome above described defects, a timing of gain change for the AGC processing at an amplifier is selected within a time interval where valid data does not exist in a received data stream according to the present invention.
Namely, a digital signal to be received by a digital information signal receiver includes a valid interval and invalid interval in the received data stream. In order to avoid signal distortions in the recovered signal, gain change in the AGC processing of the digital information signal receiver is done within the invalid interval where the valid data does not exist.
According to the present invention, the gain control for the information receiving apparatus that receives information signal having valid time interval and invalid time interval comprises: amplification means for amplifying the received information signal; means connected to the amplification means for processing output signal of the amplification means; means coupled to the amplification means for controlling gain of the amplification means; and means coupled to the gain control means for controlling a timing of gain setting of the gain control means at interval where said processing means does not process the output signal in said valid time interval in the received information data.
In the gain control of this invention, the timing control means executes the gain setting of the gain control means within the invalid time interval.
Further in the gain control of this invention, the gain control means holds gain of the amplification means during the valid time interval of the received information data.
In the another aspect of the present invention, the gain control of this invention further comprises: means coupled to the amplification means for comparing signal level of the output signal and a reference level to generate an error signal; and means coupled to the comparing means for deriving low frequency components of the error signal, wherein the low frequency components of the error signal being supplied to the gain control means.
In a more specific application of this invention, the gain control for the broadcast signal receiving apparatus that receives broadcast signal having valid time interval and invalid time interval comprises: amplification means for amplifying the received broadcast signal; means connected to the amplification means for processing output signal of the amplification means; means coupled to the amplification means for controlling gain of the amplification means; and means coupled to the gain control means for controlling a timing of gain setting of the gain control means at interval where said processing means does not process the output signal in said valid time interval in the received broadcast signal.
Further in the gain control for the broadcast signal receiving apparatus, the timing control means executes the gain setting of the gain control means within the invalid time interval included in the received broadcast data.
Further in the gain control for the broadcast signal receiving apparatus that receives broadcast data, the gain control means holds gain of the amplification means during the valid time interval of the received broadcast data.
The gain control for a broadcast signal receiving apparatus of this invention further comprises: means coupled to the amplification means for comparing signal level of the output signal and a reference level to generate an error signal; and means coupled to the comparing means for deriving low frequency components of the error signal, wherein the low frequency components of the error signal being supplied to the gain control means.
More specific aspect of the present invention, in the gain control for a broadcast signal receiving apparatus, the broadcast signal is a data comprised of plurality of carriers having orthogonal relations each other, and the processing means includes the Fast Fourier Tansform processing.
This invention further propose a method for controlling a gain of an information receiving apparatus that receives information data having valid time interval and invalid time interval.
The method of this invention comprises the steps of: amplifying a received information signal; processing output signal of the amplification step; controlling gain of the amplification step; and controlling a timing of gain setting of the gain control at interval where the processing step does not process the output signal in the valid time interval in the received information signal.
In the method of this invention, the timing control step executes the gain setting in the gain control step within the invalid time interval.
Further in the method of this invention, the gain control step holds gain of the amplification step during the valid time interval of the received information signal.
In accordance with the present invention, distortions due to gain change in the AGC system of the signal receiver are avoided without using high grade digital signal processing. Namely, the timings of the gain change are done within intervals where valid data does not exist in order to avoid the distortions due to the gain change.